440 CHEMICAL ENGINEERING
couple. If the temperature for several centimeters along the axis of the tube at the inner end is practically uniform, the depth of immersion is satisfactory. If, however, the temperature falls rapidly in this region the immersion is not deep enough. Second, the couple previously standardized is mounted complete in the fixed installation and compared with another checking couple mounted along the side of the permanently installed couple. The checking couple must have a small cross-section and must be used either unprotected or protected by an extremely thin tube in order to minimize the loss of heat by conduction. The hot junctions of this couple and of the couple under test are brought closely together but not in contact. The checking couple should indicate a temperature more nearly that of the furnace than the temperature observed with the fixed couple. If the temperature difference between the two couples is large a greater depth of immersion should be employed. These methods are complicated by local variations in the temperature of the furnace but checks of this nature, although somewhat unsatisfactory, are better than no checks at all.
Couples Purposely Insufficiently Immersed.—It is frequently desirable purposely to immerse the couple to an insufficient depth. In many processes the furnace is operated at such a high temperature that a thermocouple or protecting tube cannot withstand the severe conditions to which it may be subjected. In this case the couple may be immersed only part way through the furnace wall, or to a distance flush with the inner wall of the furnace. The temperatures indicated by couples installed in this manner are always lower than those of the furnace interior, but they bear a fairly definite relation to the temperature of the furnace, and hence the method is satisfactory for temperature control and reproduction of furnace conditions from day to day.
Protecting Tubes for Thermocouples.—The choice of a proper protection tube for a thermocouple is nearly as important as the selection of the material for the couple. Among others the following properties of a protection tube should be. considered in reference to the particular process for which the tube is required:
(a) Low porosity to gases: Many tubes become very porous to furnace gases at high temperatures. Furnace gases usually attack the couple.
(&) Low volatility: Certain metal tubes are undesirable at high temperatures because the metal distils upon the couple, thus altering its calibration.
(c) Ability to withstand high temperatures.
(d) Ability to withstand sudden changes in temperature.
(e) Ability to withstand mechanical shocks and strains.
(/) High rigidity or viscosity: Protecting tubes frequently deform and exhibit the phenomenon of plastic flow at high temperatures.
(g) Thermal conductivity: High thermal conductivity is frequently desirable when rapidly changing temperatures are measured. Usually, however, low thermal conductivity is desired so that the flow of heat along the tube is as small as possible.
(h) Ability to resist corrosion from molten metals or furnace gases.
Excellent protecting tubes are obtainable for many different industrial processes. However, for certain processes, satisfactory tubes have not as yet been developed. The problem is one for the ceramist or metallurgical engineer. This is particularly true in obtaining suitable tubes for molten metals, especially iron and brass. The following describes the more important protecting tubes now in use industrially.
Fused quartz or silica tubes are used as insulators through which the wires of a rare-metal couple are threaded, and as outer protecting tubes. The molten quartz may be drawn and worked like glass or the material may be ground and pressed, with a suitable binder, into the proper form and fired in a manner similar to that employed for the manufacture of porcelain. These tubes are translucent like china. Trans-